This invention relates to a device for carrying out focus detection by utilizing the fact that the contrast signal of the image of an object reaches a maximum value when the correct focus is obtained.
The optical intensity of each spatial frequency of the image of an object, or the power spectrum, reaches a maximum value when the focus is obtained and the degree in variation of the power spectrum is most significant in a high spatial frequency range. A focus detecting device based on this characteristic has been proposed in that art. In this prior art device, the optical intensity signal in the high spatial frequency range of an object image is extracted, so that, when the signal thus extracted is at a maximum, the focus is detected. For instance, an automatic focussing device has been provided, in which an image object is mechanically or electrically scanned by using a photoelectric transducing element to convert the spatial brightness distribution of the object image into a time series signal. Then, the high frequency components of the waveform thereof are extracted by using a differentiation circuit or a high-pass filter. The lens is held in position when the output signal thereof is at its maximum value.
Recently, a method of utilizing a self-scanning type photoelectric element has been proposed as such a scanning technique. In this known method, a self-scanning type photoelectric element is disposed in the image forming plane of the lens, so that the spatial brightness distribution of the object image is converted into a discrete time series signal. This signal is converted into an analog signal by using a sample and hold circuit and a smoothing circuit. Then the analog signal is subjected to differentiation or the like to detect the focus. However, this conventional method is disadvantageous in that, because a low-pass filter is included in the smoothing circuit, the necessary high frequency components are eliminated. As a result, it is difficult to reproduce an analog waveform equivalent to the brightness distribution of the object image, and the circuit itself is unduly large.